Remember ‘Oumuamua? The mysterious object from interstellar space baffled astronomers and made others believe they had seen an actual spaceship. First detected in October 2017 by the Pan-STARRS1 telescope in Hawaii, ‘Oumuamua, whose name means “scout” or “messenger” in Hawaiian, was traveling at an extraordinary speed of 196,000 miles per hour (315,431 kilometers per hour) and had an unusual elongated cigar-like shape that was unlike any other known asteroid or comet since it had no bright coma or dust tail. It is like no other comet scientists had seen before.

Now, a group of astronomers from the University of California, Berkeley think they have found the answer to the mysteries surrounding ‘Oumuamua, and the answer seems rather simple.

A comet traveling through the interstellar medium basically is getting cooked by cosmic radiation, forming hydrogen as a result,” said Jennifer Bergner, a UC Berkeley assistant professor of chemistry who studies the chemical reactions that occur on icy space rocks. “Our thought was: If this was happening, could you actually trap it in the body, so that when it entered the solar system and it was warmed up, it would outgas that hydrogen? Could that quantitatively produce the force that you need to explain the non-gravitational acceleration?”

‘Oumuamua is believed to have originated from another star system and is the first known interstellar object to pass through our Solar System. It was estimated to have been roughly 71 x 69 x 12 feet (115 x 111 x 19 meters). That size distinguished it from all other well-studied comets. It was so small that its gravitational deflection around the sun was altered by a tiny push created when hydrogen gas erupted from the ice.

‘Oumuamua: an alien spacecraft?

Most comets are essentially dirty snowballs that approach the sun periodically from the Solar System’s outer reaches. When the Sun heats a comet, the comet ejects water and other molecules, creating a bright halo or coma and often tails of gas and dust. The ejected gases act as thrusters on a spacecraft, giving the comet a small push that slightly alters its trajectory relative to the elliptical orbits of other solar system objects, such as asteroids and planets.

When it waws discovered, ‘Oumuamua lacked that coma or tail and was too small and distant from the sun to capture sufficient energy to eject much water, prompting some astronomers and the general public to speculate wildly about its composition and what was pushing it outward.

Was it a hydrogen-releasing iceberg? A large, fluffy snowflake pushed by the sun’s light pressure? A lightweight sail designed by an alien race? A spacecraft with its own propulsion? (Most scientists still believed that ‘Oumuamua was a natural object that has a unique shape and composition). Some possible explanations for its elongated shape include a collision with another object or tidal forces from its parent star.

One of the most intriguing aspects of ‘Oumuamua was its high speed, suggesting some unknown force accelerated it. Some scientists have proposed that ‘Oumuamua was pushed by solar radiation pressure or by gas outgassing from its surface, but subsequent observations have challenged these explanations.

Hydrogen outgassing: the explanation hiding in plain sight

Bergner, however, found that experimental research published in the 1970s, 1980s and 1990s demonstrated that when ice is struck by high-energy particles comparable to cosmic rays, an abundance of molecular hydrogen is produced and trapped within the ice. In actuality, cosmic rays can penetrate tens of meters into ice, converting up to a quarter of the water to hydrogen gas.

“What’s beautiful about Jenny’s idea is that it’s exactly what should happen to interstellar comets,” said colleague Darryl Seligman, now a National Science Foundation postdoctoral fellow at Cornell University. “We had all these stupid ideas, like hydrogen icebergs and other crazy things, and it’s just the most generic explanation.”

Bergner believed that hydrogen released from ice could be sufficient to accelerate ‘Oumuamua. As both an experimentalist and a theorist, she investigated the interaction of ice chilled to five or 10 degrees Kelvin, the temperature of the interstellar medium (ISM), with the ISM’s energetic particles and radiation.

She discovered numerous experiments demonstrating that high-energy electrons, protons and heavier atoms could convert water ice into molecular hydrogen, and that the comet’s snowball-like structure could trap the gas within the ice. Her experiments found that when heated, such as by the sun’s light, the ice anneals — or changes from an amorphous to a crystalline structure — forcing the bubbles to escape and releasing hydrogen gas.

Bergner and Seligman calculated that ice on the surface of a comet could emit enough gas, in the form of a collimated beam or fan-shaped spray, to affect the orbit of a small comet like ‘Oumuamua.

“For a comet several kilometers across, the outgassing would be from a really thin shell relative to the bulk of the object, so both compositionally and in terms of any acceleration, you wouldn’t necessarily expect that to be a detectable effect,” Bergner said. “But because ‘Oumuamua was so small, we think that it actually produced sufficient force to power this acceleration…The main takeaway is that ‘Oumuamua is consistent with being a standard interstellar comet that just experienced heavy processing. The models we ran are consistent with what we see in the solar system from comets and asteroids. So, you could essentially start with something that looks like a comet and have this scenario work.”